Delivery of therapeutic compounds to a subject is limited by the ability of these compounds to reach and enter targeted organs, tissues, and cells. Delivery of many therapeutic molecules has been found to be facilitated by lipid based delivery vehicles. Lipid-based systems generally involve liposomes or lipid particles. Lipid systems can consist of an aqueous interior surrounded by a lipid bilayer. Lipid particles and lipid bilayers consist of a hydrophobic region and a region that interfaces with water. The hydrophobic region generally consists of alkyl chains of the lipids. The polar region generally consists of charged or polar head groups of the lipids. Lipids form these structures spontaneously because of their amphiphilic character.
Liposomes or lipid particles transport therapeutic molecules in their interior, either in aqueous or hydrophobic regions, and/or on their surface. Liposomes or lipid particles may stabilize a therapeutic molecule both before and after administration to subject organisms. Liposomes may also facilitate delivery to the target tissue or organ, either systemically, e.g., in the circulation, or locally, e.g., when administered topically. Liposomes or lipid particles may also facilitate entry of a therapeutic molecule into the cytoplasm of target cells. The mechanism underlying these phenomena are incompletely understood.
Polynucleotides are an important class of therapeutic molecules. These are generally either DNA or RNA, but include a variety of modified forms, including single- and double-stranded regions. Delivery of polynucleotides, particularly double stranded RNA, is enhanced by lipid systems, either in the form of liposomes or lipid particles. Lipid particles and liposomes are capable of complexing with polynucleotides. The interaction between lipids and polynucleotides can be based on a charge complex. That is, each nucleotide of a polynucleotide carries a negative charge of a phosphate group. If the lipid is positively charged, it may complex with a charge group of a polynucleotide. Similarly, polynucleotides may complex with the polar region of a lipid bilayer. Liposomes and lipid particles may encapsulate polynucleotides or complex with them on their polar or charged surface.
Even though such lipid-polynucleotide particles have shown such great promise, little is known about how lipids and liposomes facilitate delivery of polynucleotides. While certain liposomal compositions have been found to be favorable for transport of polynucleotides to target cells, results are unpredictable, and may vary according to the process of producing the lipid particles or liposomes, and between one preparation and another made by the same process. The efficiency of delivery by a liposomal preparation varies substantially between organs. Efficient delivery in cultures of cells in vitro cannot predict whether the same preparations will enhance delivery when administered to an organism in vivo.
Cationic lipids of the lipid particles and liposomes have been found to be critical for their ability to enhance delivery of therapeutic molecules in general, and polynucleotides in particular. The bases for the critical role of cationic lipids in these systems is incompletely understood and structure function relationships are unknown; their enhancing affect may stem from one or more factors, including the ability of cationic lipids to complex with polynucleotides, their effect on the lipid bilayer structure, their ability to fuse with plasma membranes of the target cell, and/or their ability to facilitate entry of the polynucleotide into the cytoplasm of the cell. This uncertainty creates further unpredictability in this field of research.
The number of naturally occurring cationic lipids is limited. Further, the cost of natural lipids can be substantial, particularly when linked to a large-scale manufacturing process under regulatory conditions. This is because natural lipids are generally extracted and are heterogeneous, and are not synthesized. Homogeneous preparations of single natural lipids are now available commercially, albeit in limited numbers and limited to research use. However, design of novel cationic lipids has shown promise of improving delivery of lipid-based vehicles. There exists a continuing need to identify classes of cationic lipids that enhance lipid-based delivery systems, particularly ones that allow formulations with therapeutic molecules to be readily and reproducibly manufactured.